general question not sure where to post

Hi There, I've had this issue for many years that my heart will suddenly start to beat extremely rapidly and hard out of nowhere. It just happened to me a minute ago as I was just checking my email. I was at one point diagnosed with Mitral Valve prolapse but then that diagnosis was reversed the last time i had a test done on my heart. the doctor just said it was nothing and shrugged it off as no big deal but it is concerning to me. it hadn't been happening for awhile but now has been happening more frequently lately. any ideas what could cause this?